1. Field of the Invention
The present invention relates to a method and apparatus for arbitrating access to a common resource when access requests are issued from a plurality of origins.
2. Description of the Related Art
Conventionally, in a computer, when access requests such as those for reading and writing are issued from a plurality of blocks (access request origins) to a common resource such as a dynamic random access memory (DRAM), it is necessary to execute arbitration to the access requests. There has conventionally been an apparatus that executes such arbitration. The apparatus is configured to prevent the arbitration result from being unbalanced by counting the number of times of request acceptance for each of access request origins to control such that the counted numbers are equalized among the access request origins, or by a round-robin scheme.
For example, a bus arbiter apparatus is known that includes a group setting unit that makes settings by assigning each of a plurality of modules to any one of a plurality of arbitrating groups, an in-group arbitrating unit that arbitrates bus accesses to a common bus for each of the arbitrating groups, an intra-group arbitrating unit that arbitrates bus accesses to a common bus among the arbitrating groups, and a bus-use permission signal generating unit that provides a bus use permission signal to a module arbitrated by the intra-group arbitrating unit (for example, Japanese Patent Application Laid-Open Publication No. 2003-271545).
Moreover, such a bus arbiter apparatus is also known that includes a unit that groups request origins each of which needs to maintain the time from the issuance of a bus use request to the bus use permission to be short and request origins of requests other than requests for refreshing the DRAM as one group and regards the group as one request origin; a unit that, representing the lowest priority as the request of a group and the request having a priority higher by one level than the lowest priority as the request of refreshing in the initial state, changes, when a request origin for a request that is not the request of the group and not the request of refreshing has received bus use permission, the priority level of the request origin that has received the bus use permission to the lowest rank and, when the request of the group or the request of refreshing has received bus use permission, changes the lowest rank request of the priority to the request of the group and the request having the higher priority by one level than the lowest rank to the request of refreshing; and a unit that issues bus use permission in response to a bus use request of a request origin based on the priority levels changed by the above unit (for example, Japanese Patent Application Laid-Open Publication No. H11-39255).
Furthermore, such a arbitration control system is also known that includes a plurality of requesters that transmit use requests for a common resource retained commonly among a plurality of apparatuses; a control unit that receives the use requests and transmits the received use request at a predetermined frequency having been set to the requesters that have transmitted the use requests; and a arbitrating unit that arbitrates by selecting requesters to permit the use of the common resources according to the received use requests (for example, Japanese Patent Application Laid-Open Publication No. 2002-197050).
However, in the above conventional methods in which the number of times of request acceptance is counted, a counter is necessary for each of the access request origins. Therefore, the circuit scale of the arbiter apparatus tends to be large.